120v dc to 120v ac?

Question

i have a system that runs on 120v ac and has a battery back up. when i found it it was not functioning. I found it with 10 12v batteries wired in series that fed a 30A 2 pole breaker. From the breaker it goes through a bridge rectifier to a normally closed relay that is opened with ac power. the idea being if power was lost the battery power would work. I have 125v dc to the bridge rectifier but nothing coming out the other side. I have done some research and found that the rectifier doesnt take dc to ac. so i am at a loss. it was also being charged by a 24v charger, but when i turn it on it trips after 5 seconds. any help would be appreciated. and i am not sure exactly what it powers when there is an outage. i am about to trace out wires and see.

Answer 1

The rectifier connected to the batteries could serve several functions. It would prevent back-feeding AC to the battery input if the relay does not pull in when AC power is present. It would provide the same DC polarity to the load regardless of which way the batteries are connected. It would allow an alternate AC source to be connected in place of the batteries.

It is strange that you find 120VDC on the input to the rectifier and nothing on the other side. The rectifier may have failed. When rectifiers fail, they are usually shorted internally, but the short circuit current could cause the rectifier to melt open like a fuse. Look for a hole, crack or deformation of the rectifier package. Look for soot. Disconnect the rectifier and test it with an ohmmeter.

Is there some kind of reconnection scheme that reconnects the batteries in pairs to allow the 24 volt charger to charge them?

Edit

Going on the assumption that this is something that never worked. The batteries might be connected to the DC side of the rectifier. That would mean no output on the AC side. It might have been taken apart and put back together wrong or built by someone that didn't know what they were doing.

Edit 2

In my initial assumption, the main AC input would be shorted if the relay fails to pull in when AC is applied. The short circuit protection on the AC side would be the only thing protecting against a catastrophic failure. Another possibility is that the main input supplies a rectifier and power is always used as DC by the load.

Re: additional info

If DC is applied to the AC side of the rectifier, the output would be the same voltage minus about 2 volts forward drop in the pair of diodes that are conducting.

Upon further review:

The relay is not likely to pull in quickly enough to prevent being damaged by AC applied to the DC side. Therefore, rectifiers on both inputs is the only configuration that seems viable.

Answer 2

where did you get this system. It is very strange! From your description, someone started to build this, realized they were over their head, and quit.

You cannot use diodes to convert DC to AC. the reverse, yes.

As was said in your previous questions, you can get a 120 Dc to 120 AC inverter. from the 30 amp number, you need a 3600 watt inverter, quite a big one. It may have to be special order and quite expensive. Probably more than $10000

A 24 volt charger will not charge a 120 volt string of batteries. You will need a special charger, perhaps a specially designed charger.

You do realize that having unsealed auto batteries indoors is a health hazard?

Here is what I would do. Rewire the batteries for 24 volts with 5 parallel strings, each with 2 in series. This will work with your battery charger. If the batteries are old (check they with a specific density checker) scrap them all except the best 2 to get 24 volts.

Buy a 24 volt 3600 watt inverter. This you can get off the shelf. Not cheap but a lot cheaper than a 120 vDC one would be. About $1000-2000

You may want to get a smaller model.

Examine carefully how the resulting AC is connected to your house wiring. Do it wrong and you will have a major disaster! This is not to be taken lightly! You can't just have it wired in parallel. You need a special switch.

You mentioned a relay. That may work. There would be a period when the mains power came back when both would be connected at the same time. Hopefully this 5ms period is not enough to damage anything. I'd put separate fast-blow fuses in the inverter output for safety.

One other possibility is to use the 120 VDC just to run things that will work on DC, such as incandescent lamps (not CFLs). But incandescent lamps are about all that you can use.